Power line communication (PLC) techniques provide an efficient way of using power lines in a power distribution system as data transmission lines. Typically PLC systems provide broadcast-type transmission to a large number of receivers. In such PLC systems data transmission is typically realized using the differential mode of transmission.
In the differential mode a high frequency data signal is superimposed onto the standard 50 Hz or 60 Hz alternating current (AC) signal such that one wire, e.g. the phase line, carries the data signal and the other wire, e.g. the neutral line, carries the inverse of the data signal. Hence, the data signal is symmetric with respect to ground so that the sum of the signals is zero. Differential mode PLC provides good signal propagation and relatively low losses due to electromagnetic radiation.
PLC may also be used for dedicated data transmission applications. In contrast with open broadcast PLC applications, dedicated data transmission applications typically require the data communication to have a closed character in the sense that data communication is only possible between a predetermined number of devices connected to the power lines.
One problem relates to provide effective and reliable isolation between the devices associated with such closed-circuit PLC system and other devices connected to the power lines. Typical differential mode isolation filters require very large inductors and capacitors which are expensive, have large dimensions and/or have a large reactive power component.
Another problem relates to providing a power source for devices connected to such closed-circuit PLC system. The mains may be used as a power source, however this would require the use of a power supply (usually a switched mode power supply) in the electrical device. Many applications require the device to be kept small and cost efficient. The standby energy loss in such a small, low-cost power supply however may be considerable. Moreover the aggregate loss from a large number of such devices may be unacceptable.
Hence there is a need in the prior art for improved methods and systems for providing closed-circuit power line data transmission.